Time of flight ranging systems are commonly used in level measurement applications, and are referred to as level measurement systems level measurement systems determine the distance to a reflector (L.e. reflective surface) by measuring how long after transmission of a burst of energy pulses, an echo is received. Such systems typically utilize ultrasonic pulses, pulse radar signals, or microwave energy signals.
Pulsed radar and microwave-based level measurement systems are used over ultrasonic-based systems in many applications because of their improved resolution and minimized susceptibility to outside interference. As compared to ultrasonic pulse systems, radar pulse systems by their very nature deal with high frequency signals and pulse widths in the nanosecond range. To accurately and reliably process these high frequency signals using electronic circuitry, various techniques are employed.
Equivalent time sampling is one such technique. In an equivalent time sampling system, it is possible to process signals that are beyond the frequency range of conventional data acquisition systems provided the signals are repetitive. The signal needs to be repetitive because only one part of the real time signal is measured in every cycle. The portion of the signal which is sampled every cycle can be random, but then the sample needs to be stored in memory so that the whole cycle can be reconstructed after enough samples have been acquired. If the repetitive signal is sampled at a frequency which is slightly less than the repetition frequency, the samples will slowly progress along the length or span of the signal and the reconstruction can take place without storage except for the period between the samples and this can be done using a capacitor. This is further illustrated in FIG. 4, where a single cycle 401 of a repetitive waveform 400 is reconstructed as a waveform 402 by sampling the repeated waveform 400 over a number of cycles. The repetitive waveform 400 is generated by a transmit pulse signal or clock 404 and the waveform 400 is sampled by a sample clock 406 which has a frequency slightly less than the transmit clock 404.
In a radar-based level measurement system, a transmit pulse is coupled to a transducer (i.e. an antenna) to output a radar pulse. The radar pulse is reflected by a reflecting surface and the resulting echo pulse (i.e. reflected pulse) is coupled by the antenna and converted into a receive pulse for further processing by a receiver module in the level measurement system. The receiver module includes a sampling circuit which performs equivalent time sampling. As described above, the equivalent time sampling technique involves sampling the receive signal within a narrow time window. The time window is slewed back and forth by a ramp generator to search for detectable portions of the reflected pulse. The ramp generator sweeps the time window so that the sampled signal can be reconstructed. The exact delay of the receive or echo pulse from the transmit pulse that produces a detectable reflection is a measure of the distance to the reflecting surface.
The accuracy of equivalent time sampling depends in large part on accurately controlling the delay between the two clocks, i.e. the transmit timing pulse signal (the pulse train 404 in FIG. 4) and the receive timing pulse signal (the pulse train 406 in FIG. 4). Known approaches for controlling the timing fall into two general categories. The first approach relies on the use of phase locked loop (PLL) circuitry to accurately control the delay between the two signals. The second approach relies on the use of a delay circuit with closed loop control.
For a phase locked loop implementation, the accuracy of the system depends on the accuracy and temperature stability of the PLL components utilized in the system. In practical systems, this means the use of precise and often expensive PLL components.
The closed loop control of the delay between the two clocks reduces errors, however, there will be electronic components which are outside the control loop, in particular, the reference or control voltage for the ramp generator. The output of the ramp generator is a ramp signal which sweeps the time delay window for sampling the receive pulse. If the linearity or rate of change (i.e. slope) of the ramp signal changes, then the apparent velocity of the equivalent time signal would also change and the result would be incorrect distance readings.
Accordingly, there remains a need for a time-base generator with a compensating control loop which reduces the effects of temperature and time variances while providing a practical design for implementation and manufacturability.